This invention relates to control of fuel injection during a period when a catalytic converter is not heated yet in a direct injection engine provided with an injector which injects fuel directly into a combustion chamber.
The type of direct injection engine having a fuel injector which injects fuel directly into the combustion chamber is known in the prior art. Unlike other conventional arrangement in which an injector is provided in an intake passage, the direct injection engine does not cause a problem of fuel condensation on passage walls. Therefore, it provides excellent stability and response characteristics with respect to air-fuel ratio control. In addition, if the combustion chamber is designed in such a shape that an air-fuel mixture is locally distributed around a spark plug when the fuel is injected in the latter half of each compression stroke, it is possible to make the mixture leaner (higher air-fuel ratio) as a whole by using so-called stratified charge technology and thereby achieve an improvement in fuel economy.
Exhaust gases from automotive engines, for instance, contain hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). There is a demand today to reduce generation and release of these harmful constituents as much as possible to obtain improved properties of automotive emissions. One approach that has conventionally been practiced is to provide a catalyst in an exhaust passage, as is widely done in the direct injection engine.
However, catalysts used for converting the exhaust gases can not fully exhibit their conversion effects in a cold-start condition when the catalysts are at a temperature lower than their activation temperature and, therefore, considerable quantities of HC and NOx constituents are likely to be released if it takes a long time for the catalysts to reach the activation temperature.
A device intended to provide means for overcoming this problem is disclosed in U.S. Pat. No. 5,655,365, for example. This device delays the ignition timing of at least one cylinder to a point beyond its top dead center and increases the ratio of fuel to a higher level than required when the engine is in its normal operating condition while a catalyst is not heated yet. As a result, the exhaust gas temperature is increased, so that heating of the catalyst is promoted. When applied to a direct injection engine, the device injects a specified amount of fuel completely during a particular period before the top dead center (e.g., 60.degree. to 80.degree. before the top dead center, hereinafter referred to as BTDC) in each compression stroke.
The device disclosed in the Patent Publication promotes quick light-off mainly by retarding the ignition timing while supplying an excess amount of fuel during the period when the catalyst is not heated yet, and in controlling fuel injection from an injector in the direct injection engine, the device injects the fuel only in each compression stroke. If fuel injection is controlled in this manner, however, the fuel economy significantly deteriorates when engine load increases and the amount of injected fuel increases, for example, although sufficient effects are not obtained with respect to the reduction in HC and NOx emissions and increase in the exhaust gas temperature. Thus, there is left room for improvements in this device.